1. Technical Field
This invention relates generally to transmission of full-motion, television-quality color video signals and associated data and audio signals over ordinary unshielded twisted pair (UTP) telephone wiring and, more particularly, to an apparatus and method for bidirectionally transmitting such signals over a telephone wire without interfering with normal telephone signals on the wire.
2. Related Information
Unshielded twisted pair (UTP) wiring is used in many office buildings to transmit voice grade signals and low-speed data such as that generated by modems. Such wiring normally runs between 20 and 2,000 feet from a central switch or entry point to a particular telephone. A private branch exchange (PBX) is often used to connect the building wiring network to an external set of telephone lines provided by a telephone company.
Beyond an office building, however, telephone companies typically restrict the bandwidth of a particular outside "line" to no more than about 4 KHz, severely hampering the ability to transmit anything other than very low bandwidth data over long distances. As is self-evident, a typical NTSC television signal requiring approximately 6 MHz of bandwidth cannot be directly transmitted over such lines. Some companies have attempted to compress and otherwise manipulate various types of video signals to fit within the limited bandwidth of telephone wires. A representative example of such a system is disclosed in U.S. Pat. No. 5,164,980 to Bush et al. (video telephone system).
Within an office building, however, the telephone wiring is not artificially limited in bandwidth, but instead suffers from the inherent transmission qualities of the wiring such as phase shifts and attenuation at higher frequencies. Thus, significantly higher data transmission bandwidths may be achieved within an office building or other structure having similar wiring arrangements. However, the wiring transmission effects can cause drastic changes in received waveforms. These effects, while quite tolerable in voice communications, destroy the integrity of individual pulses of a baseband video signal. For example, attempting to directly transmit a baseband NTSC video signal over UTP wiring in a building would result in a wholly unacceptable result (and would also interfere with normal telephone data). Amplitude modulating such a signal (as is done in TV transmitters) also would result in an unacceptable result. As shown in the following Table 1, the typical attenuation loss as a function of frequency along 2,000 feet of UTP wire is severe:
TABLE 1 ______________________________________ FREQ (MHz) LOSS (dB) ______________________________________ 5 38 10 58 15 72 17 76 20 82 ______________________________________
Various schemes have been developed in an attempt to overcome the above problems and others. For example, some schemes involve separating various components of an NTSC signal (such as a luminance signal and a chrominance signal) and separately modulating and transmitting them. One example of such a scheme is illustrated in U.S. Pat. No. 4,955,048 to Iwamura et al. However, such schemes often require complex and expensive signal processing circuits and may be particularly sensitive to device tolerances.
Video teleconferencing systems are known in which two-way visual and audio communication is possible between individuals or groups at different locations. Video conferencing systems, however, require expensive equipment, high bandwidth communication channels, and fiber optic or other high bandwidth wiring throughout a building. High quality video conferencing over existing telephone wiring has heretofore been difficult, expensive, or impossible in many cases. For these reasons, a need exists for providing an inexpensive teleconferencing method and apparatus for use within buildings which have ordinary telephone wiring, without requiring special wiring or expensive transmission equipment.